Paper - Development of the Otic Capsule 1

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Bast TH. Development of the Otic Capsule I. Resorption of the cartilage in the canal portion of the otic capsule in human fetuses and its relation to the growth of the semicircular canals. (1932) Arch. Otolaryng. 16:19

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This historic 1930 paper by Bast described fetal otic capsule development. It is the first of a series of related papers on the development of the otic capsule (1, 2, and 3).

See also the later series on development of the stapes, fissula ante fenestram and associated structures (1, 2, 3, and 4).

Also by this author: Bast TH. Ossification of the otic capsule in human fetuses. (1930) Contrib. Embryol., Carnegie Inst. Wash. 121, Publ. 407, 53-82.

Bast TH. Blood supply of the otic capsule of a 150 mm (C.R.) human fetus. (1931) Anat. Rec. 48: 141-151.

Bast TH. Development of the Otic Capsule I. Resorption of the cartilage in the canal portion of the otic capsule in human fetuses and its relation to the growth of the semicircular canals. (1932) Arch. Otolaryng. 16:19

Bast TH. Development of the otic capsule II. The origin, development and significance of the fissula ante fenestram and its relation to otosclerotic foci. (1933) Arch. Otolaryng. 18(1):

Bast TH. Development of otic capsule III. Fetal and infantile changes in fissular region and their probable relationship to formation of otosclerotic foci. (1936) Arch. Otolaryng. 23: 509-525.
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F Declau, W Jacob, W Dorrine, B Appel, J Marquet Early ossification within the human fetal otic capsule: morphological and microanalytical findings. J Laryngol Otol: 1989, 103(12);1113-21 PubMed 2614225

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Development of the Otic Capsule I. Resorption of the cartilage in the canal portion of the otic capsule in human fetuses and its relation to the growth of the semicircular canals

T. H. Bast, Ph.D. Madison, Wis.

(1932)

From the Department of Anatomy, University of Wisconsin. These studies were undertaken for the Research Council of the American Otological Society.

Introduction

In the description of figure 29 in an earlier account of “Ossification of the Otic Capsule in Human Fetuses,”[1] reference was made to a peculiar process of resorption of part of the cartilaginous otic capsule, in the canal region, by means of vascular connective tissue buds without any attempt at ossification. Again in a paper on the “Blood Supply of the Otic Capsule,”[2] the arteries were enumerated and traced that are concerned in the formation of the vascular connective tissue buds responsible for the resorption of the massive cartilaginous capsule in the region of the semicircular canals. In his excellent injection preparations Dr. George Shambaugh saw the numerous vessels in the canal region of the otic capsule in calf fetuses and pictured them in his paper on “Verbindungen zwischen den Blutgeféissen in dem membranosen Labyrinth und dem Endosteum und den Gefassen in der knochernen Labyrinthskapsel.”[3] Resorption of cartilage that is not immediately followed by ossification is extremely rare in the normal human body. As a rule, when vascular connective tissue buds enter cartilage the cartilage is ready to be replaced by bone. Preparatory to ossification the cartilage cells enlarge and their matrix becomes calcified. Vascular buds enter this changed cartilage, resorb it and replace it by bone. This sequence holds for the greater part of the otic capsule, as described in my previous paper on ossification of the capsule, except for the massive part of the capsule in the region of the semicircular canal. The object of this account, therefore, is to describe more fully the development of the canal portion of the capsule from its cartilage stage through the stage of partial resorption and vascularization to its ultimate state of ossification.


Figures 1 to 11 are photographs of horizontal sections through the region of the subarcuate fossa (middle of the superior semicircular canal) of the otic capsule of fetuses ranging from 25 to 370 mm. in crown-rump length, corresponding to the ages of from 8 weeks to full term. This series is given to show the growth of and changes in the otic capsule, but more especially to show the changes that occur in the cartilage between and surrounding the semicircular canals and its subsequent ossification. C., cartilage; U., utricle; V., vestibule; 5a., saccule; Co, cochlea; 5., subarcuate fossa; 1.5., lateral sinus; 1.6., lateral semicircular canal; s.c., superior semicircular canal; 3, ossifica.tion center 3; 5, ossification center 5: 10, ossification center 10; N .V., Vestibular nerve; l.c.a., lateral canal ampulla.


Fig. 1. Embryo 87, slide 18, section 3; size, 25 mm. At 5. the perichondrial connective tissue is dipping into the cartilage to form the subarcuate fossa. An enlarged view of part of this section is shown in figure 12; X 5.


Fig. 2. Embryo 86, slide 15, section 3; size, 50 mm. This shows a marked growth in the capsule over that in the 25 mm. embryo (fig. 1). The extent of growth can be seen by comparing the distance between the outer margin of the superior semicircular canal (sac) and the utricle (U.) in the two figures. The vascular connective tissue in the subarcuate fossa (5.) has extended, and in the cartilaginous capsule (C.) just ahead of the subarcuate fossa is seen an area of degenerating cartilage. For an enlarged View of this see figure 13. A few blood vessels from the vascular connective tissue of the subarcuate fossa have already penetrated this degenerating cartilage; X 5.


Fig. 3. Embryo 22, slide 24, section 4; size, 100 mm. The span between utricle and superior canal is increased. The degenerating cartilage is replaced by vascular connective tissue from the subarcuate fossa (5.). Other vascular buds are seen deeper in the cartilage; X 5.


Fig. 4. Embryo 57, slide 17, section 7; size, 112 mm. The condition is much the same as in figure 3. An advance in growth is indicated. A few spots of degenerating cartilage are noted. Note the much lighter stain of the cartilage preceding its excavation by vascular connective buds; X 5.


Fig. 5. Embryo 5, slide 38, section 310; size, 135 mm. The spread between the utricle and the superior canal is increased. The cartilage is greatly excavated by vascular connective tissue from the subarcuate fossa. The bone seen at 3 is part of ossification center 3; X 5.

Fig. 6. Embryo 13, slide 37, section 221; size, 161 mm. The vascular buds have penetrated deeper into the cartilage. Although a number of the ossification centers are present, there is no ossification initiated by the vascular buds in the cartilage; X 5.

Fig. 7. Embryo 21, slide 46, section 1; size, 183 mm. At this stage the vascularization of the cartilage has reached its height. The cartilage is being ossified from ossification centers, but the vascular connective tissues play no part in the process, although the ossifying process is beginning to surround them; X 5.


In his excellent account on the “Histogenesis and Growth of the Otic Capsule and Its Contained Periotic Tissue—Spaces in the Human Embryo,” Streeter[4] summarized the previous accounts of the develop'nent of the internal ear and traced the development of the otic capsule from the mesenchyma through the precartilage to the cartilage stage, carefully noting the growth changes. Special emphasis was laid on the changes in the capsule in relation to the growth and development of the semicircular canals, vestibule and cochlea.

While it is not the intention in this paper to cover any of the ground covered by Streeter, it was found necessary to start with embryos as small as 25 mm. in crown—rump length, or about 8 weeks old, in order to trace the complete process of removal and ossification of the massive cartilaginous portion of the otic capsule which surrounds, lies between and is included within the arcs described by the three semicircular canals.

Material and Methods

These observations are based on serial sections of the ears of fifty~two human fetuses ranging from 25 mm. to 370 mm. in crown-rump length, which corresponds to the fetal ages of from 8 weeks to full term. Wax plate reconstructions were made of the ears of human fetuses 14 weeks old (100 mm. in crown-rump length), 16% weeks old (126 mm.), 21 weeks old (183 mm.) and 34 weeks old (310 mm.) to show the growth and extent o-f the vascular connective tissue buds within the cartilagenous capsule and their subsequent replacement by bone.


The petrous portion of the temporal bone containing the otic capsule was removed from the fetuses, fixed, decalcified (when necessary) in an alcoholic solution of 5 per cent nitric acid, embedded in parloidin (except the ears of the two youngest fetuses, which were embedded in paraffin), sectioned in series and stained with hematoxylin and eosin.

Observations

The outstanding change in the cartilaginous otic capsule in the canal region preceding ossification is the process of cartilage resorption and replacement by vascular connective tissue. This process uniformly begins at the surface of the mass of cartilage included within the arc described by the superior semicircular canal, and is accomplished by resorption of the cartilage and its replacement by a vascular connective tissue ingrowth from the overlying perichondrium and dura. In some animals the flocculus of the cerebellum extends into this excavated pocket, in which case it is known as the floccular fossa. In man and other higher mammals in which the flocculus does not extend into the excavation, it is known as the subarcuate fossa. The site of the sub— arcuate fossa is therefore the starting point for the process of partial resorption of the cartilaginous capsule in the canal zone. From the start it should be borne in mind that this resorption process is absolutely independent of and different from the resorption process that immediately precedes ossification.


In figures 8 to 11 progressive stages in capsular growth are shown. In figures 8 and 9 only a little cartilage is left. The vascular connective tissue buds have been in part replaced by bone, but some are still present surrounded by bone. In figures 10 and 11 the entire capsule is ossified and the vascular connective tissue buds can hardly be distinguished from the bone marrow. The connective tissue of the subarcuate fossa is receding but still present. It is being ossified from its margin. fig. 8.—Embryo ()2, slide 31, section 2; size, 215 mm; X 5. fig. 9.—Ernbryo 64, slide 35, section 1; size, 230 mm.; X 5. fig. 10.—Embryo 60, slide 43, section 3; size, 305 mm; X 5. fig. 11.——-Embryo 67, slide 48, section 3; size, 370 mm.; X 5. figures 12 to 21 are photographs from sections through the otic capsules of fetuses of various ages to show in more detail the vascular connective buds, their method of excavating the cartilage, the details of cartilage degeneration preceding the entrance of the vascular bud and the ossification of cartilage an.d vascular buds.

fig. 12. Embryo 87, slide 18, section 3; size, 25 mm. This is a higher power magnification of part of figure 1 to show the vascular connective tissue bud forming the subarcuate fossa at .5‘. The cartilage in the region of the canal is very young and shows a transition stage between precartilage and cartilage; X 47.

fig. 13. Embryo 86, slide 15, section 3; size, 50 mm. This is a higher power of part of figure 2 to show the cartilage necrosis at CU, and the connective tissue in the subarcuate fossa at 5.; X 47.


In a 25 mm. embryo, which is the youngest in this series, the site of the subarcuate fossa is clearly indicated by the increased amount of vascular perichondrium and dura, which already dips into the cartilage (figs. 1 and 12). In a 50 mm. embryo this condition is more marked and, in addition, there is a spot of degenerating cartilage just ahead of the ingrowing vascular connective bud (figs. 2 and 3, C. u.). A few capillary vessels have already entered the necrotic area. Such necrotic cartilage areas (figs. 14 to 17) are frequently seen in the capsule of the canal region in fetuses up to 161 mm. in crown—rump length. They precede some but not all of the ingrowing vascular buds. In the case of the posterior canal region they are constant and attain a considerable size before vascular buds enter them. The exact nature of these necrotic areas has not been determined, but the process seems to be one of mucoid degeneration. In the younger fetuses these areas are definitely circumscribed (figs. 14, 15 and 16,. d. c.). In somewhat older fetuses, as shown in figure 17 d. c., there are diffuse areas around the main areas. The main area in the 146 mm. fetus which figure 17 depicts lies in the arc of the posterior canal. It is very large and is hemorrhagic. The process in both stages seems to be one of cell necrosisand liquefaction of the cartilage matrix. Immediately following this process blood vessels enter the cartilage and replace it by vascular connective tissue. Many of the vascular connective tissue buds, however, penetrate into and remove cartilage without being preceded by this necrotic process. The necrotic process seems to be important, especially in the regions of the posterior and superior canals, in initiating and speeding up the early vascularization of the cartilage. After the connective tissue buds have a good start they are able to progress without the aid of the necrotic process. Thus after the seventeenth week of intrauterine life no necrotic areas have been found in the region of the superior canal. In the region of the posterior canal the condition is a little different. Here the necrotic areas are more prominent and the vascular connective tissue is less active; that is, cartilage degeneration goes on much further (up to the twentieth week) and involves larger areas before the Vascular buds from the posterior meningeal and recurrent mastoid arteries enter. The buds formed by these arteries are not as large as those formed by the subarcuate artery in the region of the superior canal.


Fig. 14. Ii1nl:-r_\'u 3, slizle 13. section 5: size. 100 mm. ln the lower part of this figure vascular connective tissme buds are seen entering the Cartilage. At top center necrotic cartilage is seen at (I.('. In the upper arm the (leg'e11eratim1 is not as Complete as in the lower. Ntiiiierotis necrotic nuclei can still be seen: X 59.

Fig. 15. Embryo 5 (a. “slide 21, .~;eetiun .3; size, L30 mm. In the upper ri5_:;l1t—l1and corner is a largzge necrotic area. Tliis is more atlvzmeerl than in figtire 22 and has the El])1)CZ11'a11C€ of encapsulation. The \:a;<.e.1_1lar emineetix-'e tissue l')11(lS seen at lower left are not very active. for they areeiicapsulated; ,\< 59. ’

Fig. 16. Embryo 12. slide .39, seetimi 3; size. 135 mm. The necrotic area at the upper left is definitely eirCu1n.~‘.cril)e(l and well (legencrated: at the upper right and at t_he bottom are vascular connective tissue l_mr_l_~; (el'.I>.l which are quite active and are not encapsulated. This is especially true of the l<:)\\'er one: X 59.

Fig. 17. Embryo 30. slide 34. seetiun 6; size. 146 mm. This is an area of diffuse necrotic eartila_<.:‘e. Such areas are fmintl armiml the main necrotic areas especially in the region of the


Fig. 18. Embryo 32, slide 24. section 4; size, 100 mm. This slmws vascular buds in a youn_e,r utie capsule. .-\t the l)(JttuI1 of the picture a group of young vascular buds are shown. There is very little eo1111ecti\'e tissue arotintl the vessels, but the cartilage immediately stirroulltling them has stained faintly. At the trip is an older hurl with :1 great deal of Ct)1]1lCCtlVC tissue. lts border is quite cellular, giviiig the appearance of capsule furmatioii. There is still a rim of Imurl}-' stained L‘£iI"[ilEl_5;{C stirrmiiitliiig‘ the hurl: X 47.

Fig. 19. Embryo 21. slide 43, section 1; size. 183 nmi. In the lower half of the picture the vascular buds are well eiieapsillatetl. There is still a rim of poorly stained cartilage. At the upper right, bone is forming frrnn an ossification Center. At the upper left the cartilage cells are enlarged preparatory to resorption and ossification. The upper rim of the vascular bud (l".!>.) adjacent to the ussifieatitm center is already partly ossified, but ()ssificatim1 is not initiated by the Vasctllar buds: x 47.

Fig. 20. Embryo ()9, slide 48. section 3: size 205 111111.; X 47.

Fig. 21. Embryo 67. slide 35. section 2: size, 370 mm. In figiires 20 and 21 the vascular chzmnels are surrounded by bone. In figure 20 the vascular connective tissue Content is still ehar


In the 100 mm. fetus (fig. 3) the vascularization from the subarcuate artery has extended a considerable distance into the cartilage and has involved the necrotic area shown in figures 2 and 13. In figures 4 to 6 the progressive steps in the process of vascularization are seen. The height of the process is reached about the twentieth week of intra-uterine life or a little earlier. This is the time when ossification around the superior and posterior canals is taking place. The entire vascularization process is accomplished by the subarcuate, recurrent mastoid, posterior meningeal and accessory stylomastoid arteries, as already described by me? Several buds from the recurrent mastoid artery are shown at the upper left of figure 7. In figure 6 theilvascular buds are seen pushing into the center of the arc of the lateral semicircular canal. The story of the growth and extent of the vascular buds is told more clearly by the series of models shown in figures 27 to 33. In a 100 mm. fetus (figs. 27 and 28) the vascular connective tissue in the subarcuate fossa is already prominent, and from it a few small buds penetrate the cartilage between the canals. A vascular bud is penetrating the capsular cartilage from the capsular perichondrium. In figures 29 and 30 the subarcuate tissue is massive and many buds are present. A few smaller buds are seen in different parts of the capsule. One eventually extends into the loop of the lateral canal and is derived from the accessory stylomastoid artery. One grows into the cartilage of the loop of the posterior canal, and one develops from the recurrent mastoid artery and aids the subarcuate in excavating the cartilage between the canals. The stereoscopic views in figures 32 and 33 show both the anterior and posterior aspect of a model of the vascular connective buds at the height of their development in a fetus 21 weeks old. The vascular connective tissue (72. C.) has profusely penetrated the cartilaginous capsule in the region of the canal. After this age the advancing bone formation involves both the cartilage and the contained vascular tissue. Thus in a 34 week old fetus (fig. 31) only the connective tissue in the subarcuate fossa and a few buds in the remaining patches of cartilage are left. The rest of the capsule is bone.


Figure 34 is an anterior view of a model of the internal ear of a 150 mm. human fetus showing the arterial supply to the otic capsule. It is given here to show the prominent subarcuate artery. This artery arises with the internal auditory artery, gives a few branches to ossification center 10 and then passes into the connective tissues of the subarcuate fossa. It is responsible for the vascularization of the greatest part of the cartilage in the region of the canal. This figure also shows the rich venous plexus around the endolymphatic sac and its drainage into the lateral sinus.


Figs. 22, 23 and 24. Embryo 5, slide 37, section 2; size, 135 mm. At the upper right of figure 22 a vascular connective tissue bud is shown invading the cartilage. At 112' . a monocyte is sending a pseudopodium into a degenerating cartilage cell (C). In figure 23 cartilage is seen at the lower left. The upper part of the picture shows part of a vascular connective tissue bud containing a large number of monocytes. figure 24 shows a group of monocytes (M .) in a vascular bud‘ X 425.


Fig. 25. Embryo 40, slide 24, section 2; size, 155 mm. In older vascular buds, as’ in this 155 mm. fetus, the monocytes QM.) are quite large and stain faintly, and the cytoplasm has many vacuoles. After this age the monocytes are scarce; )( 425."


Fig. 26. Embryo 41, slide 25, section 8; size, 160 mm. The top of this picture shows enlarged cartilage lacunae with necrotic cartilage cells. In the middle of the picture the matrix between these lacunae is darker staining, indicating calcification. In the lower part of the picture the ossification process is invading the changed cartilage. At M. a group of monocytes similar to those in the vascular buds are removing the necrotic cartilage cells. In the very lowest part osteoblasts and monocytes are in the lacunae; X 425.


The beginning of the ossification of the cartilage in the canal region is shown in figure 5 at 3. Additional ossification centers 5 and 10 are shown in figures 6 and 7. The cartilage around the canals and vestibule ossifies first and then the process proceeds and encroaches on the vascular connective tissue buds. In figure 7 the bone around the lateral canal ampulla (l. c. as.) is beginning to surround the vascular buds in the cartilage. The cartilage containing the vascular connective tissue is thus gradually replaced by bone, as shown in figures 8, 9, 10 and 11, but the ossifying process is never initiated by the connective tissue buds. In a 215 mm. and a 230 mm. human fetus the region of the canal is ossified except for a few areas where cartilage persists (figs. 8 and 9). Many of the vascular connective tissue channels can still be seen. They gradually are replaced by bone and bone marrow. In the last month of fetal life the entire capsule is ossified and the connective tissue in the subarcuate fossa becomes more or less replaced by bone (fig. 11).

Histologic Considerations

There are certain histologic features in the vascularization process of cartilage that are of interest. They are illustrated in figures 18 to 26. figures 18 to 21 show in successive steps the origin and fate of the vascular buds. Very early buds are shown at the bottom of figure 18. Here terminal capillary loops are surrounded by a light staining cartilage. Connective tissue has not yet developed around the vessels. The cartilage preceding and surrounding these vascular loops has undergone some chemical change so that its matrix does not take the hematoxylin stain as does the rest of the cartilage matrix. At the top of the same figure is an older bud. The vessels here are surrounded first by a loose mesenchymal tissue, which later becomes quite dense. At the periphery of the bud the connective tissue cells are grouped to give the effect of capsule formation. The cartilage immediately surrounding this is poorly stained, like that around the young buds. This poorly stained cartilage is present around the vascular buds up to the time of ossification. Late vascular buds just preceding ossification are shown in figure 19. Ossification is advancing from the upper right. Immediately preceding ossification the cartilage lacunae enlarge, the cartilage cells become necrotic and the matrix stains more deeply with hematoxylin, indicating calcareous deposits. The ossification process is encroaching on the upper end of the vascular bud (72. b.). The interesting fact is that although ossification of the bud is taking place the cartilage immediately surrounding it is still poorly stained and the lacunae do not enlarge. This indicates that the cartilage around these buds is nonactive and necrotic and is resorbed without further change. It is of further interest that the vascular buds become ossified from advancing ossification centers, but never initiate ossification. After being surrounded of the vascular connective tissue growth into the cartilaginous capsule of the Canal region.


Figs. 27 and 28. Ernbryo 22; size, 100 mm. figure 27 is the anterior and figure 28 the posterior view. They show the vascular connective tissue plug in the subarcuate fossa and small buds extend into the capsu

depict models of four human fetal ears at different ages, showing the extent e between the canals.


Figs. 29 and 30. Embryo 11; size, 126 mm. figure 29 is the anterior and figure 30 the posterior view. There is a marked increase in the vascular connective tissue.

Fig. 31. Embryo 68; size, 310 mm. This is a posterior view. There is a large plug of connecThe rest of the capsule is ossified and most of the Connective tissue buds have been removed by the process.

tive tissue in the subarcuate fossa.

by bone, vascular buds retain their identity for some time (72. b., fig. 20), but gradually they become invaded by bone trabeculae and the mesenchymal tissue is replaced by bone marrow, as shown in figure 21.


Considerable difficulty was encountered in trying to determine what happened to the cells of the cartilage which was replaced by the vascular connective tissue. In the connective tissue of the young invading buds there are numerous cells with round chromatic nuclei and a cytoplasm which ranges from a uniformly granular to a vacuolated type. At first it seemed that these cells were liberated cartilage cells. After careful search it was found that at the tips of the advancing buds cartilage cells were necrotic (fig. 22 C.), and that these cells acted as phagocytes to remove the dying cartilage. At M. in figure 22 one of these‘ cells, which we consider as monocytes or histiocytes, is attacking a necrotic cartilage cell (C.). In the upper two thirds of figure 23 a vascular bud with many histiocytes is shown, and at the lower right some of the surrounding cartilage. The younger histiocytes have a granular cytoplasm (fig. 24), while the older ones have a more vacuolated cytoplasm (fig. 25) and often a pyknotic nucleus. Sometimes young histiocytes are seen surrounding a necrotic older histiocyte, apparently in the act of removing it. By comparison it was noted that these histiocytes are almost identical in appearance with the cells that invade the enlarged cartilage lacunae and remove the necrotic cartilage cells preceding ossification. This is shown in figure 26. At C. is a necrotic cartilage cell in an enlarged lacuna. At M. a group of histiocytes have destroyed the cartilage cell and inhabit the enlarged lacuna. The other cells in the bottom part of the picture are osteoblasts which will deposit bone when sufficient excavation has been accomplished.


The histiocytes in the vascular connective tissue seem to disappear about the time that ossification centers begin to ossify the canal portion of the otic capsule. It was also noted that the removal of cartilage occurs only at the tips of the advancing buds and not along their sides. As a matter of fact, their sides are separated from the surrounding cartilage by a cellular capsule, as shown in the older buds in figures 18 and 19 and also along the sides of the bud in figure 22.


It appears, therefore, that in the process of vascularization there is not as great a destruction of cartilage as the massive vascular connective tissue ingrowth would indicate. There is just enough cartilage destroyed to permit the blood vessels to penetrate. The connective tissue of the bud then grows around the vessels, but not at the expense of the surrounding cartilage, moving the cartilage ahead of it en masse. Thus the rapid and massive growth of the capsule in the region of the canal is due not entirely to cartilage growth but in a large degree to the growth of the vascular connective tissue processes. The significance of this procedure will be considered further under the discussion of growth and expansion of the semicircular canals.



Figs. 32 and 33. Embryo 21; size, 183 mm. Anterior and posterior stereoscopic views. At this stage the vascular connective tissue has reachec. the height of its growth in the cartilaginous capsule. Ossification, rapidly advancing in the subsequent ages, ossifies the remaining cartilage and this connective tissue. Thus in the ear of the 310 mm. fetus shown in figure 31 most of the connective tissue has been ossified: .s'.c., superior canal; l.c., lateral canal; ;,‘w.c. posterior canal; a.c., aqueductus cochlea; f.a.f., fissura ante fenestrarn; N.VlI, facial nerve; 5., stapes; v.c., vascular connective tissue in cartilage; e.s., endolymphatic sac; c.s., connective tissue in subarcuate fossa; NJ/III, auditory nerve; N.p.c., nerve to posterior canal ampulla; N.l.s.c., nerve to lateral and superior canal ampullae.

Growth and Expansion of the Semicircular Canals

The major growth of the otic capsule and of the semicircular canals, as already indicated by the sections in figures 1 to 11, takes place between the eighth and twenty—first weeks of intra—uterine life. At 8 weeks the canals are encased by an immature cartilage, which reaches maturity shortly after. This means that the tremendous growth and expansion of the canals must take place within this firm tissue. Among the numerous accounts of the growth of the internal ear, Streeter’s is the most exhaustive. He has shown that the cartilaginous capsule continues to grow and expand as long as the contained canal spaces continue their growth, and that the canals change position also with respect to the cells of the encasing cartilage. This is accomplished by a retrogressive differentiation of the cartilage along the greater curvature of the canals, and a building on of cartilage along the lesser curvature. Thus in the growing cartilaginous capsule there is a building on of cartilage at the periphery of the capsule, a destruction of cartilage along the advancing margin of the canals and a building along the receding margin of the canals.


Size of the Semicircular Canals of Human Fetuses of Various Ages’?

Crown— Rump

Embryo No.1‘ Length, Mm. Superior, Mm. Posterior, Mm. Lateral, Mm. 87 ............................. .. 25 0.8 0.9 0.55 90 ............................. .. 43 1.45 1.25 1.05 86 ............................. .. 50 1.3 1.55 1.1

100 2.6 2.8 1.7 3 ............................. .. 100 3.0 2.65 2.25

111 3.1 2.45 2.7 53' ............................. .. 112 3.3 2.85 2.3 57 ............................. .. 112 3.3 2.2 2.15 54 ............................. .. 115 3.25 2.95 2.2 55 ............................. .. 117 3.6 3.05 2.2 56 ............................. .. 120 3.8 3.9 3.0

126 3.6 3.1 1.95 135 3.75 3.1 2.2 12 ............................. .. 135 4.1 3.6 2.8

5 ............................. .. 135 4.25 3.7 2.5 37 ............................. .. 140 3.7 3.0 2.0 30 ............................. .. 146 4.5 4.8 3.15 38 ............................. .. 147 4.45 3.6 3.05 39 ............................. .. 150 4.85 4.5 3.3 40 ............................. .. 155 4.2 4.1 2.6

160 4.15 4.1 2.6 13 ............................. .. 161 4.5 4.1 3.45 33 ............................. .. 163 4.8 4.75 3.2

180 4.1 5.01 2.6 45B .......................... .. 180 3.7 5.0 2.8 21 ............................. .. 183 4.6 5.25 3.45

190 4.3 4.0 2.75 14 ............................. .. 202 4.2 3.9 3.6 70 ............................. .. 202 4.45 4.75 4.15

7 ............................. .. 205 4.1 4.9 4.3 51 ............................. .. 210 4.65 5.18 3.3 62 ............................. .. 215 4.9 5.3 4.05 46 ............................. .. 222 4.55 4.4 2.5

2 ............................. .. 230 4.3 5.0 3.6 64 ............................. .. 230 4.4 4.4 3.2 42 ............................. .. 240 5.0 4.9 3.0 30 ............................. .. 246 4.3 ..... .. 3.0 15 ............................. .. 246 3.9 4.5 3.6

260 4.1 5.1 2.9 69 ............................. .. 265 4.5 4.85 4.0

4 ............................. .. 275 3.9 5.1 3.2 66 ............................. .. 290 4.85 5.1 3.25 59 ............................. .. 290 4.25 4.55 3.35 20 ............................. .. 295 3.9 4.55 2.9 60 ............................. .. 305 4.55 5.1 3.0

310 4.6 5.4 3.25 61 ............................. .. 345 ..... .. 4.3 ..... .. 16 ............................. .. 363 4.0 5.35 3.25

370 4.75 5.8 4.2 1 ............................. .. Full Term 4.1 4.0 3.0

The figures represent the actual diameters of the lesser arcs in the horizontal ‘ plane. 1‘ Embryos 29A and 29B, and 45A and 45B were two sets of twins.


In the light of the observations presented here, it is apparent that the vascular connective tissue buds must be considered as another factor in the growth of the otic capsule and the contained canals. The massive connective tissue ingrowth of the subarcuate fossa at the center of the arc of the superior canal, the ingrowths of the recurrent mastoid, the posterior meningeal and accessory stylotnastoid arteries and the associated necrosis of cartilage occur at the time when the canals begin their rapid expansion. These processes so honeycomb the cartilaginous capsule that the canals with their surrounding cartilage are able, at least in part, to expand en masse. In this expansion the remaining cartilage cells of necessity must enlarge and multiply, and regressive cartilage dedifferentiation and cartilage building, as described by Streeter, must still go on to make possible the greater arcs, but the vascularization process reduces this cartilage growth to a minimum between and in the arcs of the canals.


Fig. 34. Embryo 39; size, 150 mm. This is a drawing of part of a model of the internal ear of a human fetus about 18% weeks old. It shows the arterial blood supply to the Capsule. It is designed especially to show the subarcuate artery, but it also shows the rich venous plexus around the lower part of the endolymphatic sac and its drainage into the lateral sinus. The ossified part of the otic capsule is shown, as are also, 1, 2, 3, 8 and 10, ossification centers from which ossification started; sac. e:v2.dol., saccus endolymphaticus; v. plexus, venous plexus; lat. sinus, lateral sinus; can. post, posterior canal; cam. sup., superior canal; can. let, lateral canal; a. to ossi. cent. 10, artery to ossification center 10; -v. vestibularis, vein to the vestibule; 0:. sub. arcu., subarcuate artery; aquaed. cock, aquacductus cochlea; ram. cock. at. coch. int, cochlear branch of the internal cochlear artery; mm. west. a. cash. int, vestibular branch of the internal cochlear artery.


Measurements of the canals of a series of fetuses ranging from 8 weeks of age to full term show that the greatest growth of the canals takes place in the first half of intra-uterine life. These measurements are given in the table and shown in the graph (fig. 35). The tabulated figures are the actual diameters of the lesser arcs of the semicircular canals. The measurements were taken in a horizontal plane both from serial sections and fro-m wax plate reconstructions.


These figures show that the diameter of the canals almost doubles between the crown—rump lengths of 25 mm. and 50 mm., more than doubles between those of 50 mm. and 100 mm., and continues to increase until the size of about 150 mm. to 200 mm., when the approximate maximum growth is reached. The three canals do not reach their maximum size at the same time. The superior canal matures first in fetuses of about 150 to 160 mm., the posterior next in fetuses of about 180 mm. and the lateral a little later. This fact was observed in my account of ossification of the otic capsule,‘ and at that time was associated with the appearance of ossification centers in the capsule around the canals. Ossification begins in the capsule of the superior canal at about the time that it reaches its maximum diameter. Ossification of the posterior canal occurs a little later and that of the lateral canal last. In the graph the growth curves of the three canals is clearly shown. The curve is practically flat after the maximum growth is reached.


In a recent article entitled “Ueber Wachstumsprozesse in der normalen Labyrinthkapsel und ihre Beziehungen zur Otosklerose,” Werner[5]claimed that the growth of the canals continues during the first years of postnatal life. His observations were made on chickens and also on children. In his photomicrographs of sections of the internal ear it appears as though the bone was resorbed at the periphery of the canal arcs and built on at the lesser curvature. I have also noted this condition in older human fetuses. The amount of bone laid on at the lesser curvature, however, is so small that it would be difficult to measure. There are, no doubt, growth changes in the otic capsule with the growth change of the petrous bone during early childhood, for the shape of the adult petrous bone is quite different from that of the fetus or very young child. In the present series of human fetal models it appears that the overall horizontal length of the internal ear is longer in full term fetuses than in the half term fetuses, in which the canals, according to my figures, are full size. It is this expansion of the entire internal ear that may account for the histologic pictures that Werner presented. Measurements of the canals of children or adults would have to be made and compared with those of fetal canals before one could definitely say whether or not there is a postnatal growth of the canals. But since I have seen bone changes around the canals in older fetuses like those shown by Werner, although from my measurements it is apparent that the canals of the full term fetus are no larger than those of 200 mm. fetus, one must conclude that the bone changesvaround the canals are due not to actual growth of the canals but possibly to expansion of the entire internal ear.



Fig. 35. Graph showing the sizes of the semicircular canals of human fetuses plotted against the crown-rump lengths. The arrows indicate the approximate size of the fetus when the canals reach their maximum growth.


However, this matter must be left open for the present. From the measurements given here, however, one thing is certain: The canals reach their approximate maximum growth at about half term. If there is any growth in size beyond this stage it is so small that the measurements do not show it.


There is, however, a difficulty in drawing absolute conclusions from the measurements given in the table and graph because the size of the canals varies in different fetuses of approximately the same age. Take, for instance, the posterior canal in the 202 mm. fetuses: In one it measures 3.9 mm., and in the other, 4.7 mm. As one glances over the graph it will be noted that in some cases there are variations of more than 1 mm. in the diameter of the canals in fetuses of approximately the same size. However, the mean growth line is horizontal from the half to the full term stage.

Summary

  1. The growth of the semicircular canals takes place within a mass of cartilage which surrounds, lies between and is included within the arcs of the semicircular canals. The expansion of the canals involves two processes: (a) an extensive destruction of cartilage between and Within the arcs of the canals by necrosis and extensive connective tissue ingrowths, and (b) a regressive dedifierentiation of cartilage at the advancing margin and a building of cartilage at the receding margin of the canals.
  2. Removal of cartilage is accomplished by monocytes or histiocytes which enter the cartilaginous capsule with the ingrowing connective tissue buds.
  3. The canals reach their approximate maximum growth at about half term or at the time when their capsule begins to ossify. The exact time varies in the three canals. The superior canal reaches its maximum growth in fetuses of about 150 to 160 mm. in crown-rump length; the posterior, in fetuses of about 180 mm., and the lateral, in fetuses of about 200 mm.

References

  1. Bast TH. Ossification of the otic capsule in human fetuses. (1930) Contrib. Embryol., Carnegie Inst. Wash. 121, Publ. 407, 53-82.
  2. Bast TH. Blood supply of the otic capsule of a 150 mm (C.R.) human fetus. (1931) Anat. Rec. 48: 141-151.
  3. Shambaugh, George E.: Verbindungen zwischen den Blutgefassen in dem membranosen Labyrinth und dem Endosteum und den Gefassen in der knochernen Labyrinthskapsel, Ztschr. f. Ohrenh. 50:327, 1905.
  4. Streeter GL. The histogenesis and growth of the otic capsule and its contained periotic tissue-spaces in the human embryo. (1918) Contrib. Embryol., Carnegie Inst. Wash. 8: 5-54. Streeter, George L.: The Histogenesis and Growth of the Otic Capsule and Its Contained Periotic Tissue—Spaces in the Human Embryo, Contrib. Embryol. (no. 20) 7:5, 1918.
  5. Werner, C. F.: Ueber Wachstumprozesse in der normalen Labyrinthkapsel und ihre Beziehungen zur Otosklerose, Arch. f. Ohren-, Nasen- u. Kehlkopfh. 129:128 (June 26) 1931.



Cite this page: Hill, M.A. 2017 Embryology Paper - Development of the Otic Capsule 1. Retrieved November 18, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Development_of_the_Otic_Capsule_1

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